The present invention relates to a radiation detector provided with a scintillation element optically coupled to photodetectors for detecting the axial position of radiation incident to the scintillation element, and, more particularly, to a method of optimized position resolution by varying the light reflection characteristics of the scintillation element.
Conventional radiation detectors of the scintillator type have at least one scintillator comprising a scintillation crystal and a pair of photodetectors optically coupled to the scintillation crystal. Radiation entering into the crystal (for example, gamma-rays, neutrons, other high-energy particles), is changed into light at each point by a process known as scintillation. A large fraction of the light is transmitted to the photodetectors at either end of the crystal, resulting in the production of photoelectrons which are multiplied to yield a measurable current pulse for counting purposes. On the basis of measured and calculated comparisons of the relative outputs from the photodetectors, it is possible to ascertain the incident position of the radiation entering the crystal. An example of such a conventional radiation detector is illustrated in FIG. 1 and represented generally by the numeral 10.
In FIG. 1, radiation is shown entering the scintillation crystal 15 and causing scintillations at emitting point P therein. Photodetectors 20 and 30 are optically coupled to either end of scintillator 15, for example, by a transparent material such as silicon grease (not shown). Scintillation 15 is a pillar-shaped crystal of length L, and radiation is illustrated to be incident on position X.
The well known principle of position-dependent light collection associated with the detector of FIG. 1 is shown in FIG. 2, where the intersecting curves show the relationships of relative outputs A(x) and B(x) of photodetectors 20 and 30, respectively, as a function of position x (or X) of the incident radiation. Outputs Ax and Bx are attributed to light emission caused by scintillation at point P, the intersecting curves representing the fraction of light reaching the photomultipliers 20 and 30. Using conventional means (not shown), the incident position X may be determined therefrom by dividing one output by the sum of both outputs or simply comparing one output to another.
In fabricating a conventional radiation detector of the type illustrated in FIG. 1, the photodetector output responses are adjusted by methods which typically include adjusting the shape and light reflecting characteristics of the scintillation crystal. For example, crystals having a circular or tetragonal cross section may be used. Reflective coatings which have mirror-like(polished reflecting) or dispersive (disperse reflecting) characteristics may be applied to the side surfaces. An example of a mirror-like coating is aluminum foil. Barium phosphate and magnesium oxide are examples of dispersive coatings.
However, difficulties are encountered in determining the optimal combination of scintillator shape and dimensions and light reflection characteristics according to an arbitrary object, because the variation in photodetector output with respect to a change in location of the radiation incident to the scintillator is typically small. As a result, it is also difficult to obtain a high degree of position resolution in measuring the location of incident radiation.
In providing for a radiation detector with improved position resolution, it is important to provide for a photodetector output response which is sharp, that is, whose characteristic response curve of output intensity versus location of incident radiation (as illustrated in FIG. 2) possesses a steep slope, and not a gentle slope.